In the conventional silicon nitride sintered bodies, ones of which the surface is coated with ceramic material having high hardness and wear resistance in order to improve the wear resistance has been known (JP Patent Kokoku Publication No. 63-1278 (1988).
A further silicon nitride sintered body has been known in which crystal phases of both .alpha.- and .beta.-Sialons are homogeneously distributed over the entire sintered body (JP Patent Kokoku Publication No. 63-35594 (1988) etc.). Problems to be solved by the present invention:
As for the former sintered body, it is difficult to obtain a sufficient bonding force to the substrate through the difference in thermal expansion and/or chemical affinity between the coating ceramic material and the silicon nitride substrate, entailing a high cost counted as a problem in economy. Thus the former sintered body has found almost no practical application.
The latter sintered body which includes both the crystal phases of .alpha.- and .beta.-Sialons evenly distributed over the entire sintered body suffers variation in its properties as their proportion changes and cannot develop every property. Namely, if one attempts to enhance one property any of other properties is forced to be deteriorated, resulting in an intermediate performance on balance of the both.